Transmitters and receivers for communication systems generally are designed such that they are tuned to transmit and receive one of a multiplicity of signals having widely varying bandwidths and which may fall within a particular frequency range. It will be appreciated by those skilled in the art that these transmitters and receivers radiate or intercept, respectively, electromagnetic radiation within a desired frequency band. The electromagnetic radiation can be output from or input to the transmitter or receiver, respectively, by several types of devices including an antenna, a wave guide, a coaxial cable and an optical fiber.
These communication system transmitters and receivers may be capable of transmitting and receiving a multiplicity of signals; however, such transmitters and receivers generally utilize circuitry which is duplicated for each respective signal to be transmitted or received which has a different frequency or bandwidth. This circuitry duplication is not an optimal multi-channel communication unit design architecture, because of the added cost and complexity associated with building complete independent transmitters and/or receivers for each communication channel.
An alternative transmitter and receiver architecture is possible which would be capable of transmitting and receiving signals having a desired multi-channel wide bandwidth. This alternative transmitter and receiver may utilize a digitizer (e.g., an analog-to-digital converter) which operates at a sufficiently high sampling rate to ensure that the signal of the desired bandwidth can be digitized in accordance with the Nyquist criteria (e.g., digitizing at a sampling rate equal to at least twice the bandwidth to be digitized). Subsequently, the digitized signal preferably is pre- or post- processed using digital signal processing techniques to differentiate between the multiple channels within the digitized bandwidth.
With reference to FIG. 1, a prior wideband transceiver 100 is shown. Radio frequency (RF) signals are received at antenna 102 processed through RF converter 104 and digitized by analog-to-digital converter 106. The digitized signals are processed through a discrete fourier transform (DFT) 108, a channel processor 110 and from channel processors 110 to a cellular network and a public switched telephone network (PSTN). In a transmit mode, signals received from the cellular network are processed through channel processors 110, inverse discrete fourier transform (IDFT) 114 and digital-to-analog converter 116. Analog signals from digital-to-analog converter 116 are then up converted in RF up converter 118 and radiated from antenna 120.
A disadvantage of this alternative type of communication unit is that the digital processing portion of the communication unit must have a sufficiently high sampling rate to ensure that the Nyquist criteria is met for the maximum bandwidth of the received electromagnetic radiation which is equal to the sum of the individual communication channels which form the composite received electromagnetic radiation bandwidth. If the composite bandwidth signal is sufficiently wide, the digital processing portion of the communication unit is very costly and consumes a considerable amount of power. Additionally, the channels produced by a DFT or IDFT filtering technique must typically be adjacent to each other.
A need exists for a transmitter and a receiver, like the one which is described above, which is capable of transmitting and receiving a multiplicity of signals within corresponding channels with the same transmitter or receiver circuitry. However, this transmitter and receiver circuitry preferably should reduce communication unit design constraints associated with the above transceiver architecture. If such a transmitter and receiver architecture could be developed, then it would be ideally suited for cellular radiotelephone communication systems. Cellular base stations typically need to transmit and receive multiple channels within a wide frequency bandwidth (e.g., 824 megahertz to 894 megahertz). In addition, commercial pressures on cellular infrastructure and subscriber equipment manufacturers are prompting these manufacturers to find ways to reduce the cost of communication units. Similarly, such a multi-channel transmitter and receiver architecture would be well suited for personal communication systems (PCS) which will have smaller service regions (than their counterpart cellular service regions) for each base station and as such a corresponding larger number of base stations will be required to cover a given geographic region. Operators which purchase base stations ideally would like to have a less complex and reduced cost unit to install throughout their licensed service regions.
An additional advantage may be gained by cellular and PCS manufacturers as the result of designing multi-channel communication units which share the same analog signal processing portion. Traditional communication units are designed to operate under a single information signal coding and channelization standard. In contrast, these multi-channel communication units include a digital signal processing portion which may be reprogrammed, at will, through software during the manufacturing process or in the field after installation such that these multi-channel communication units may operate in accordance with any one of several information signal coding and channelization standards.
Another disadvantage of traditional communication system design is that the hardware associated with the communication system is typically dedicated to a single access method (i.e., advanced mobile phone service (AMPS), narrowband advanced mobile phone service (NAMPS), United States digital cellular (USDC), personal digital cellular (PDC) and the like communication access methods). In order to provide multiple access, i.e., access to the communication system through any of the access methods, significant hardware duplication, at considerable cost, is required. Therefore, there is a need for a communication system which provides for multiple access while not significantly increasing the amount of required hardware, and hence associated cost.
Digital signal processing is evolving as the preferred implementation in many signal processing applications. The advent of improved, higher speed and lower cost digital signal processors (DSPs) and other digital circuit elements coupled with increased flexibility and accuracy of digital circuits is driving a move to converting a number of signal processing applications from the analog forum to the digital forum. Digital signal processing, while offering the above mentioned advantages and other advantages, does not come without some drawbacks. For example, some applications, particularly in the field of radio frequency (RF) communications, are inherently analog. Signal processing for RF applications often require converting an analog signal, for example an RF or intermediate frequency (IF) signal, to a digital signal and likewise converting digital signals to analog signals. An example of such an application is in wideband digital transceivers such as shown and described in commonly assigned U.S. Pat. application Ser. No. 08/366,283, the disclosure of which is hereby expressly incorporated herein by reference.
In many digital processing applications, including those accomplished in a wideband digital transceiver, the precision of a signal must be converted from a high level of precision to a lower level of precision. For example, a signal represented as 32 bits of information may have to be reduced to a signal represented as 16 bits of information. This is due to the limited capabilities of certain digital processing elements such as, for example, digital-to-analog converters (DACs). In making such a conversion, however, there is a loss of information. One will appreciate in the above example that 32 bits can represent more information than 16 bits at a given data rate. The result of this loss of information is quantization noise.
Often the noise is distributed over the entire Nyquist bandwidth and the noise power per Hertz is negligible. However, frequently the noise appears at discreet frequencies, like second and third harmonics of the signal, which pose significant problems.
To overcome the problem of noise dwelling at particular frequencies, it has been proposed to introduce psuedorandom noise to the signal, often referred to as dithering. A number of dithering techniques are taught in U.S. Pat. Nos. 4,901,265, 4,951,237, 5,073,869, 5,228,054 and 5,291,428. A major disadvantage of dithering is the requirement of having to provide pseudorandom noise generator circuitry which is often complex making the application implementation intensive and costly.
Therefore, a need exists for a method and apparatus for reducing quantization noise without significantly increasing the cost and complexity of the digital signal processing circuit.
There are numerous advantages to implementing a radio communication system using digital techniques. Notably, there is enhanced system capacity, reduced noise, and reduced hardware and associated power consumption. Fundamental to the digital radio communication system is the requirement that the received analog radio signal be digitized. The well known Nyquist criteria provides that such digitization is accomplished with minimal error at about twice the bandwidth of the analog signal. In U.S. Pat. No. 5,251,218 a methodology typical of the prior art is disclosed for digitizing an analog radio frequency signal in accordance with this principle. It will be appreciated, however, where the radio signal occupies a large bandwidth, ADCs capable of operation at very high sampling rates are required. Such devices, to the extent they are available, are expensive and often suffer reduced performance, i.e., have significant distortion and increased power consumption when operated at high sampling rates.
The spectrum allocated to radio communication systems is typically large with respect to the requirements for digitizing. In some radio communicatiorn systems, however, although the desired signal occupies a large bandwidth, not all of the bandwidth is occupied by signals of interest. In cellular radio telephone communication systems, for example, the communication bandwidth is not contiguous. The cellular A-band, for example, is allocated a bandwidth of 12.5 megahertz (MHz). Spectrally, however, the entire A-band covers 22.5 MHz of bandwidth in two discontinuous portions. In order to digitize the A-band, one would need an ADC capable of operating, according to Nyquist criteria, at least at 45 MHz or 45 million samples per second (Ms/s), and more reliably at 56 Ms/s. Splitting the signal into smaller segments allows the use of multiple ADCs at lower sampling rates. Using multiple ADCs has the disadvantage of requiring more hardware. Furthermore, clock frequency and higher order harmonics thereof inevitably fall into the frequency band of the signal being digitized. Still another disadvantage is the amount of digital data handling required to filter, interpolate, compensate for band overlap and sum the resulting multiple digital signals.
Therefore, there is a need for a device for digitizing wideband frequency band signals which is does not require high sampling rates, and does not significantly increase the amount of hardware required for the communication system.
The many advantages and features of the present invention will be appreciated from the following detailed description of several preferred embodiments of the invention with reference to the attached drawings in which: